From the Super Mario Wiki
Jelectros are glowing, electric jellyfish that stays in one place underwater in Super Mario Bros. 3. They appear in World 3-5 and World 7-4. They cannot be defeated by any means and will hurt Mario if touched. A large swarm in the middle of World 7-4 makes a hard time for swimming, especially since the screen moves automatically. Mario should try and swim slowly when surrounded by them.
Jelectros also made appearances in The Adventures of Super Mario Bros. 3. Here they were similar to Bloopers in appearance, except they were yellow with blue near their red (or bloodshot) eyes. They appeared in episodes such as "Reptiles in the Rose Garden". Here they were aggressive and would zap Frog Mario. However, they didn't appear in many episodes.
In the Nintendo Adventure Book Leaping Lizards, Luigi spots some Jelectros through a window while exploring Iggy Koopa's secret underwater lab in Water Land. Three weeks later, some Jelectros appear in the swimming portion of the International Mushroom Games obstacle course. If the reader chooses Luigi to compete in the course, one of the Jelectros will shock and briefly stun him as soon as he enters the water.
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